The use of effective and dependable protection devices (hand shield, protective helmet) in working with electric welding apparatus has been a present need since the beginning of this technique, as is apparent from the patent literature in the form of different proposals for such protective devices. According to the general progress of the technique, apparatus with electro-automatic function have been in the foreground. With such apparatus, a light shield is changeable between open and closed condition by means of an electomagnetic system which is either mechanical or is electro-optical, by means of a depolarizer (liquid crystal), whereby the closed condition of the light shield, according to modern practice without exception, is activated by a photoelectric element responsive to the light intensity in the welding zone. While the known protecting devices are in accord with the opinion of a competent specialist, it has been found that in practice they do not work satisfactorily and are subject to many difficult disadvantages.
The most serious disadvantage to which all known devices are subject without exception is that electrical signal for controlling the light shield to its closed condition comes too late, so that the welder is subjected to light exposure which, because of its being partially in the ultraviolet range, is harmful even though it is short. In the case of welding work over a long period of time, the continual exposure to the light flashes each time the welding arc is ignited can frequently lead to irreversible eye damage. Moreover, the failure of the light shield to close sufficiently quickly can lead to mechanical damage of the eyes or to sight scars of the welder through sputtering in the weld area. Relevant statistics of the employer's liability insurance of all industrial countries gives the information that the number of such injuries with the resultant liability is disturbingly high.
A further significant disadvantage of the above-mentioned delayed control of the light shield in going to its closed condition is a possible decrease in the quality of the weld because the welder, partially through anxiety with respect to the expected light flash, and partially because of the glare, does not hold the welding electrodes in exact position and thereby the weld area becomes unclean and requires expensive corrective work.
The cause for the delayed control of the light shield lies in the above-mentioned use of a photoelectric element producing a closing signal. Such an element can, logic-wise, first respond when the arc light has already reached a harmful level and thus comes inevitably too late to close the light shield in time, quite apart from the fact that to this already delayed signal must be added the time constant for operation of the light shield elements. To explain these most difficult disadvantages of known arrangements briefly and in other words, the harmful arc light must first occur before a closing signal for the light shield can be produced. With the use of photoelectric equipment for producing the closing signal for the light shield, satisfactory operation cannot be obtained regardless of cleverness in the modification of the electrical and/or mechanical part of the protection device.
The above-mentioned serious disadvantages apply to all previously known examples in the patent literature relating to protective devices and, in particular:
1. CH-PS No. 393 632 PA0 2. CH-PS No. 562 607 PA0 3. FR-PS No. 7409611 PA0 4. DT-OS No. 2 550 559 PA0 5. GB-PS No. 834 021 PA0 6. UP-PS No. 2 423 320 PA0 7. DT-AS No. 2 315 308 PA0 8. FR-PS No. 73 10949 PA0 9. US-PS No. 3,540,058 PA0 10. DE-OS No. 2.349.794 PA0 11. GB-PS No. 1,077,096
Of these known arrangements, items 1-6 and 11, which work with a mechanical light shield, have other serious faults in addition to the above-mentioned disadvantage of a delayed closing signal. In particular, with an electromagnetic system for moving the light shield from its normal open position to a closed position, the operation is either much too sluggish by reason of the long stroke of the magnet armature when the viewing area is of a practical useful size or, alternatively, the viewing area must be reduced to an extremely small window in order to reduce the stroke of the magnet armature. The dimensioning of the electromagnet is in accordance with the following rule: If the magnet is made correspondingly large for a sufficient stroke of the light shield, it is unusably heavy and, moreover, its time contant is such that the light shield cannot be closed quickly enough. If the magnet is made correspondingly small for a small stroke of the light shield, its time constant is indeed shorter, but the force of the magnet is not strong enough for timely control of the light shield weight. These physical principles are unalterable and condemn all prior electromagnetic solutions of previously known construction and mode of operation to failure. Moreover, there is no provision for preventing reopening of the sight shield when the welding arc, during the welding operation, is extinguished for a very short time, and again struck, as is often the case by reason of irregular burning off of the electrodes, or from drawing too long an arc.
Moreover, also with known protection devices with an electronically controlled light shield, i.e., with a depolarizer (liquid crystal) controlled with an electrical voltage, there exist significant disadvantages making it impossible to guarantee satisfactoy operation even if the signal for controlling the light shield to its closed condition were not delayed. The widely held and completely understandable assumption that with the use of such electronic light shield all problems of mechanical light shields would be automatically solved, is in many respects false. In particular, such depolarizers, in any case according to the present worldwide stand of the technique, have a time constant which is far greater than the relatively short time period required for true protection of the eyes. Thus, even in the most favorable cases, the time constant of such depolarizers is approximately 80 to 100 ms, whereas, for true protection of the eyes, the time period for operation of the light shield must be less than 10 ms. The proof of this is found in the relevant technical literature, as well as in data sheets of the producers of liquid crystals and, moreover, also in recent prospectuses of producers of protection equipment of this kind. Moreover, such liquid crystal light shields do not have the necessary filtering properties, in particular against the particularly harmful ultraviolet rays of a welding arc such as those of dark glass such as normally are used in the welding technique, and, furthermore, they do not repel the likewise harmful infrared rays. On account of the much too slow reaction time of liquid crystal light shields, and their insufficient filtering and damping qualities, the following compromise has been proposed. Half of the viewing window of the protecting device is provided with a liquid crystal layer, and the other half is provided with a changeable dark glass in the usual manner. However, as the liquid crystal layer remains transparent for the above-mentioned time period of about 80 to 100 ms. after ignition of the welding arc, and hence provides no protection for the eyes, damping filter discs are provided. The whole arrangement thus has the unforeseen disadvantages, firs, that when beginning a weld, the viewing window does not have fully transparency and permits only a greatly damped view of the weld area by reason of the filter discs, and, second, after ignition of the welding arc, the welder must change his eye position from the free view zone to the dark glass, which makes necessary a new search for the welding area proper and can lead to an impairment of the weld quality. Thus, as long as there is no fluid crystal which can change its optical condition in less than 10 ms, from full transparency to a condition in which it has effective filtering properties for the harmful rays of the welding arc, such electronic protecting system can provide no optimal solution to the long existing problem.